Zadig ou la Destinée ("Zadig, or The Book of Fate") (1747) is a famous novel and work of philosophical fiction written by Enlightenment philosopher Voltaire. It tells the story of Zadig, a philosopher in ancient Babylonia. The author does not attempt any historical accuracy, and some of the problems Zadig faces are thinly disguised references to social and political problems of Voltaire's own day.

It was originally published as Memnon in Amsterdam (with a false imprint of London given) and first issued under its more familiar title in 1748.

The book makes use of the Persian tale The Three Princes of Serendip. It is philosophical in nature, and presents human life as in the hands of a destiny beyond human control. It is a story of religious and metaphysical orthodoxy, both of which Voltaire challenges with his presentation of the moral revolution taking place in Zadig himself. Zadig is one of Voltaire's most celebrated works after Candide. Many literary critics have praised Voltaire's use of contradiction and juxtaposition.

Jesrad – an angel who disguises himself as a hermit. This character is directly inspired from a mysterious character referred to in the Quran, in Sura "The Cave" (Al-Kahf), v. 60-82. Known as Al-Khadir (meaning "The Green One"), he appears as a wise man holding great knowledge of the unknown and who Moses is going to follow through a journey.

Zadig, a good-hearted, handsome young man from Babylonia, is in love with Sémire and they are to marry. Sémire, however, has another suitor: Orcan wants her for himself. Zadig tries to defend his love from Orcan's threat, but his eye is injured in the process. Semire abhors this injury, causing her to depart with his enemy. Shortly after, Zadig makes a full recovery and falls into the arms of another woman - Azora - with whom he is married, but who promptly betrays him.

Disillusioned with women, Zadig turns to science but his knowledge lands him in prison, the first of several injustices to befall him. Indeed, the conte derives its pace and rhythm from the protagonist's ever-changing fortunes which see him rise to great heights and fall to great lows. Upon his release from prison, Zadig rises in favour with the king and queen of Babylonia and is eventually appointed prime minister; in this role he proves himself to be a very honest man, looked upon favourably by the king, as he passes fair judgements on his citizens unlike the other ministers who base their judgements on the people's wealth. He is forced to flee the kingdom, though, when his relationship with king Maobdar is compromised: Zadig's reciprocated love for queen Astarté is discovered and he worries that the king's desire for revenge might drive him to kill the queen.

Having reached Egypt, Zadig kills an Egyptian man while valiantly saving a woman from his attack on her. Under the law of the land, this crime means that he must become a slave. His new master, Sétoc, is soon impressed by Zadig's wisdom and they become friends. In one incident, Zadig manages to reverse an ancient custom of certain tribes in which women felt obliged to burn themselves alive with their husbands on the death of the latter. After attempting to resolve other religious disputes, Zadig enrages local clerics who attempt to have him killed. Fortunately for him, though, a woman that he saved (Almona) from being burned intervenes so that he avoids death. Almona marries Sétoc, who in turn gives Zadig his freedom and then he begins his journey back to Babylonia in order to discover what has become of Astarté.

En route, he is taken captive by a group of Arabs, from whom he learns that king Moabdar has been killed, but he does not learn anything of what has become of Astarté. Arbogad, the leader of the group of Arabs, sets him free and he heads for Babylonia once more, equipped with the knowledge that a rebellion has taken place to oust the king. On this journey he meets an unhappy fisherman who is about to commit suicide as he has no money, but Zadig gives him some money to ease his woes, telling us that source of his own unhappiness is in his heart, whereas the fisherman's are only financial concerns. Zadig prevents him from committing suicide and he continues on his way.

Zadig then stumbles upon a meadow in which women are searching for a basilisk for their lord who is ill, ordered by his doctor to find one of these rare animals to cure his sickness. The lord has promised to marry the woman who finds the basilisk. While there, Zadig sees a woman writing "ZADIG" in the ground, and he identifies her as Astarté. His former lover recounts what happened to her since Zadig fled Babylonia: she lived inside a statue when he left, but one day, she spoke while her husband was praying before the statue. The king's country was invaded and both Astarté and his new wife were taken prisoner by the same group, and the king's wife agrees to formulate a plan along with Astarté to help her escape so that she would not have a rival for the king. Astarté ends up with Arbogad, the very same robber that Zadig encountered, who then sold her to Lord Ogul, her current master. In order to secure Astarté's release from Ogul, Zadig pretends to be a physician. He offers Lord Ogul to bring him a basilisk if he grants Astarté her freedom; instead of providing the basilisk, the lord is tricked into taking some exercise, which is what he really needs to cure him from his illness.

Astarté returns to Babylonia where she is pronounced queen before a competition begins to find her a new king. Zadig triumphs in the contest which takes place between four anonymous knights, but one of the losing competitors steals Zadig's armour before the winner is revealed, and falsely claims victory. Zadig bemoans his fate, thinking that he will never be happy.

While wandering on the banks of the Euphrates, Zadig encounters a hermit reading "the book of destinies", and he agrees to stay with him for a few days. The hermit claims that he will teach Zadig lessons in life: in one such incident, the pair go to an opulent castle and are treated well, but the hermit steals a gold sink; afterwards, they visit the house of a miser and are pushed to leave, but the hermit gives the miser the gold sink. The aim, he tells Zadig, is that the hospitable man at the castle will learn not to be as ostentatious and vain, and the miser will learn how to treat guests. In another, the hermit throws a fourteen year-old boy into a river, drowning him, as he claims that Providence tells that he would have killed his aunt within a year, and Zadig within two. The hermit then reveals his true identity as the angel Jesrad, and opines that Zadig, out of all men, deserves to be best informed about Fate. Jesrad states that wickedness is necessary to maintain the order of the world and to ensure that good survives. Nothing happens by chance, according to the angel: Zadig happened upon the fisherman to save his life, for example. Zadig should be submissive to Fate, he continues, and should return to Babylonia, advice which he follows. Surprisingly regarding Voltaire's hostility towards religions, this passage is based on one of the suras of the Quran: Sura The Cave (Al-Kahf), S. 18; V. 60-82, when Moses follows a mysterious character, endowed with great knowledge, through his journey.

On his return, the final part of the challenge to be king is taking place: the Enigmas. Having solved the Enigmas with consummate ease, Zadig proves that it was he that won the first contest. Zadig marries Astarté, is crowned king, and rules over a prosperous kingdom.

"Today, anyone who sees only the print of a cloven hoof might conclude that the animal that had left it behind was a ruminator, and this conclusion is as certain as any in physics and in ethics. This footprint alone, then, provides the observer with information about the teeth, the jawbone, the vertebrae, each leg bone, the thighs, shoulders and pelvis of the animal which had just passed: it is a more certain proof than all Zadig's tracks."[1]

T. H. Huxley, the proponent of Darwin's theories of evolution, also found Zadig's approach instructive, and wrote in his 1880 article "The method of Zadig":

"What, in fact, lay at the foundation of all Zadig's arguments, but the coarse, commonplace assumption, upon which every act of our daily lives is based, that we may conclude from an effect to the pre-existence of a cause competent to produce that effect?"[3]